memento__memento_with_inner_class.kt

class Editor {
    
    // ...

    fun backup(): Memento {
        return ActualMemento(text, caretPosition)
    }

    fun restore(memento: Memento) {
        memento as ActualMemento

        text.clear()
        text.addAll(memento.text)
        caretPosition = memento.caretPosition
    }

    // ...

    open inner class Memento

    private inner class ActualMemento(text: List<String>, caretPosition: Int) : Memento() {
        val text: List<String>
        val caretPosition: Int

        init {
            this.text = ArrayList(text)
            this.caretPosition = caretPosition
        }
    }
}